The present invention relates to methods for reducing NOx emissions in the incineration of tail gas. The present invention also relates to an apparatus for reducing NOx emissions in the incineration of tail gas.
Carbonaceous fuels and other organic material are combusted in a wide variety of industrial processes. Furnace reactors, combustion engines, combustion chambers, boilers, furnaces, heaters, hot gas generators, burners, waste incinerators, and the like, are used to combust carbonaceous fuels. This combustion equipment may be used to make energy, incinerate waste and byproduct materials, or both. During a typical combustion process within a furnace or boiler, for example, a hydrocarbon feedstock or fuel is combusted in the presence of oxygen, and a flow of a combustion exhaust gas is produced. Carbonaceous fuels can be made to burn more completely, and with reduced emissions of carbon monoxide (CO) and unburned hydrocarbons, when excess air is used. Excess air usage can permit higher flame temperatures. Unfortunately, combustion at high temperatures can generate nitrogen oxides (often referred to as NOx). Free radicals of nitrogen and oxygen in air can react at such high combustion temperatures to form thermal NOx. NOx may also be formed as the result of oxidation of nitrogen containing species in the fuel, such as those that may be found in heavy fuel oil, municipal solid waste, and coal. Emissions of NOx are controlled by laws, directives, regulations and the like at many locations of operation of combustion equipment throughout the world. An exhaust aftertreatment may be required for combustion equipment at a given location to attain lower, compliant NOx emission levels.
Previous exhaust aftertreatment techniques tend to reduce NOx using various chemical or catalytic methods. Such methods include, for example, nonselective catalytic reduction (NSCR), selective catalytic reduction (SCR), and selective noncatalytic reduction (SNCR). Such aftertreatment methods typically require some type of reactant for removal of NOx emissions. The NSCR method can involve using unburned hydrocarbons and CO to reduce NOx emissions in the absence of O2. Fuel/air ratios must be controlled to ensure low excess O2. Both reduction and oxidation catalysts are needed to remove emissions of CO and hydrocarbons while also reducing NOx. Combustion exhaust containing excess O2 generally requires chemical reductant(s) for NOx removal. Among the selective processes, SCR processes can involve passing a nitrogen oxides-laden effluent across a catalyst bed in the presence of ammonia, to achieve NOx reductions. With respect to SCR, the installation and operational costs of the catalyst system may not be economical. SNCR processes can involve the introduction of NOx-reducing treatment agents into the effluent in the absence of catalyst and elevated temperature to achieve NOx reductions. With respect to previous SNCR, concerns have been raised about problems of NH3 breakthrough and byproduct CO emissions.
In some industries, such as in carbon black production, refinery operations, or petrochemical operations, for example, exhaust gases generated in primary process units are conveyed to burners or boilers for energy production, heat recovery, or incineration. These operations can generate emissions, which can be subject to any applicable air quality controls or requirements. A furnace carbon black producing process, for example, typically employs a furnace reactor having a burner or combustion chamber followed by a reactor. A combustion gas feed stream, typically a hydrocarbon gas stream such as natural gas, or the like, is combusted in the burner portion along with an oxidant feed gas stream such as air or oxygen, to produce hot combustion gases which pass then to the reactor portion of the furnace. In the reactor, hydrocarbon feedstock is exposed to the hot combustion gases. Part of the feedstock is burned, while the rest is decomposed to form carbon black, hydrogen, and other gaseous products. The reaction products typically are quenched, and the resulting carbon black and off-gas mixture is conveyed to a bag collector or other filter system, whereupon the carbon black content is separated from the tail gas. The recovered carbon black typically is finished to a marketable product, such as, for example, by pulverizing and wet pelletizing. Water from the pelletizing typically is driven off with a dryer, which may be gas-fired, oil-fired, process-gas fired such as with tail gas, or combinations of these. The dried pellets can then be conveyed from the dryer to bulk storage or other handling. The dryer also can generate gaseous emissions. The principal source of emissions in the carbon black furnace process typically is from the tail gas. Other than direct venting, tail gas emissions have been discharged using flares. The tail gas can contain combustible gas components. The composition of the tail gas after separation of the carbon black and prior to any aftertreatment may vary according to a grade of carbon black being produced and other process parameters. The untreated tail gas from carbon manufacture typically may include combinations of particulate matter, carbon monoxide (CO), thermal nitrogen oxides, sulfur compounds, polycyclic organic matter, trace elements, and other components.
The present inventors have recognized that gases containing nitrogenous byproducts of combustion in some applications may include NOx precursors which also can pose a concern for NOx emission control. The present inventors have recognized that a tail gas stream from a furnace carbon black production process, for example, can contain fuel-derived NOx precursors which have not been previously fully appreciated or resolved. Further, the present inventors believe that methods and systems for comprehensively controlling nitrogen oxides as well as nitrogen oxide precursors in combustion effluents have not been previously developed, nor have the possible benefits of such methods and systems previously been fully realized or attainable, until the development of the present methods and arrangements.